Most of the spectrum used for cellular operation is allocated in a paired way, i.e. two FDD (frequency division duplex) equal-size frequency channels, one used for downlink (DL) transmission from a BS (Base Station) to wireless User Equipments (UE), and the other used for uplink (UL) transmission from the UE to the BS. Due to smart phone use, user traffic has become more and more asymmetrical, i.e. most of the traffic is downlink-centric, due to applications such as video streaming, file sharing and internet browsing.
The average ratio between the DL and UL traffic can reach, according to an Ericsson Mobility Report, Nov. 2012, up to 9:1 (10% overall). This asymmetry means, considering that the DL spectral efficiency is 1.5 times higher than the UL spectral efficiency, that only about 15% of the uplink channel spectrum is actually used. Even with a less aggressive asymmetry factor, such as 4:1, less than 40% of the available uplink channel is used. The total available time-frequency resources in the uplink channel spectrum may thus be 60-85% of the uplink channel bandwidth. A number of solutions combining FDD with time-domain duplexing (TDD) have been proposed. For example, U.S. application Ser. No. 13/286,209 “FDD and TDD carrier aggregation” relates to the carrier aggregation of FDD and TDD component carriers, where each carrier occupies its own frequency channels for operation.
U.S. application Ser. No. 13/208,213 “Backward compatible LTE system design for asymmetric uplink/downlink spectrum” considers a system which operates with multiple DL spectrum blocks and one up-link spectrum block.
U.S. Pat. No. 6,859,655B2 “TDD FDD Air interface” relates to the adaptation of two TDD systems such to operate on a FDD (paired) allocation.
U.S. patent application Ser. No. 12/777,945 “Dual mode radio for frequency division duplexing and time division duplexing communication modes” describes a FDD-TDD multiplexed frame structure.
U.S. Pat. No. 7,929,468, “Method for improving coexistence between adjacent TDD and FDD wireless networks” by the same inventor, refers to FDD and TDD use by equipment operating in separate adjacent frequency channels or adjacent frequency bands.